1. Field of the Invention
This specification discloses two related inventions: a first relating to cable core, and to apparatus and a method for its manufacture, and a second relating to the laying of optical fibre into grooves in such cable core.
2. Description of Related Art
The first invention relates to a cable core formed with a longitudinal channel for receiving a filament such as an optical fibre or fibre ribbons, to apparatus for extruding such a cable core and to a method of manufacturing such a cable core. The invention is particularly useful in the manufacture of optical cable incorporating an extruded core of thermoplastics material having a set of parallel longitudinally-extending channels, at least one of which accommodates one or more optical fibres, the core being surrounded by an outer sheath.
Optical fibre cable of this type must be capable of tolerating axial strain without transmitting tension to the optical fibres, and this has generally been achieved by causing the channels to twist in a continuous or periodically reversing helix about the core axis. The optical fibres are loosely accommodated within the respective channel or channels, which may either be completely closed or be formed as grooves in the periphery of the core.
Optical cable of this type must also tolerate being bent around a drum, again without tensioning or compressing unduly the optical fibres, and periodic reversal of the helical twist of the channels is capable of giving the optical cable such a tolerance.
In order to provide sufficient overfeed of the optical fibres in their respective channels to give the optical cable a sufficient tolerance of axial strain, for example 0.3%, the channels are required to twist with a much shorter pitch than is required simply to give the optical cable the necessary tolerance to bending around a drum. The wide amplitude and relatively short pitch twisting of the channels makes the subsequent process of feeding optical fibres or ribbons into the channels especially difficult, since the channels have to be followed by the feeding mechanism.